CHAPTER XIII
SLIDING, ROLLING, WALKING, AND CREEPING
IN ALL forms of transportation friction plays a most important part. If there were no such thing as friction, it would be impossible for us to set an object in motion by the means that are now commonly in use, and once the object were moving it would be impossible to stop it except by bringing it up against a wall set squarely across its path.
In transportation on land friction is a much more serious bar to motion than it is in marine or aerial transportation. There are two kinds of friction that we have to contend with; sliding friction when two contacting surfaces are dragged by each other; and rolling friction when one surface rolls upon another. It is impossible to obtain two surfaces that are absolutely smooth; there are minute elevations in each that sink into minute depressions of the other like intermeshing teeth and a grinding action takes place as these microscopic inequalities are broken away. Oil reduces the friction by filling up these inequalities, but the oil itself offers a certain amount of friction just as water does along the sides of a moving ship.